PET is a nuclear medicine procedure for imaging and measuring physiologic processes within the body. It depends upon the distribution into the body of a systematically administered radiopharmaceutical labeled with a radioactive isotope ("radioisotope") that decays through the emission of positrons. This is very distinct from other nuclear imaging techniques such as Computed Tomography ("CT") which measures the distribution of electron density, or Magnetic Resonance Imaging ("MRI") which measures the distribution of protons in the body. There are literally hundreds of possible radiopharmaceuticals that find application to neurology, oncology, and cardiology. PET is typically directed to the study of metabolism processes, blood flow, blood pooling, and receptor sites in the brain.
In accordance with PET practice, a radiopharmaceutical (sometimes termed the "labeled compound") is injected into or inhaled by a patient after he or she has been positioned properly relative to an adjacent scanner device. It is the function of the scanner device to detect the gamma-rays that are produced when positrons emitted from the radioisotope annihilate with surrounding electrons. For example, a brain metabolism study might involve the injection of a fluorodeoxy-glucose radiopharmaceutical containing .sup.18 F into the blood stream so that it is taken up in the brain at sites of metabolic activity. When an .sup.18 F nucleus decays it emits a positron which, within a distance of a few millimeters, annihilates with an electron producing two oppositely directed 0.511 MeV gamma-rays. Crystal gamma-ray detectors in the scanner device surrounding the patient's head detect the arrival of the gamma-rays and identify the paths on which they traveled, defining the lines along which the annihilation events occurred. Time-of-flight techniques may also be used to locate the position of the events along the lines. Appropriate electronic circuits and a computer system(s) acquire data during the scan and map the distribution of the annihilation events, which coincide with the presence of the radioisotope. Quantitative evaluation of the function under study, as well as an image for display, are produced as a final product of the PET scan.
Radioisotopes are presently generated by accelerating protons to an energy of 12 MeV (or deuterons to an energy of 6 MeV) with a cyclotron. This proton/deuteron beam is extracted from the cyclotron and steered to a target material. Automatic chemical processors convert the target material into basic chemical building blocks, called "precursors", needed to make the radiopharmaceuticals of interest. Some state-of-the-art systems produce the final radiopharmaceutical with the aid of a programmed robot to avoid radiation exposure to a radiochemist. The PET scanner, which resembles a CT scanner in physical appearance, along with the cyclotron, targets, and chemical processors form the basic PET system.
Unfortunately, the half-life associated with many radioisotopes of interest to PET applications is very short (on the order of minutes), hence it is not possible to manufacture the radiopharmaceuticals at a manufacturing site and transport them to a patient location. Rather, the patient must travel to the site of the PET system where the needed radioisotopes can be produced and used immediately. Because of the sheer size, mass and expense of building and operating just the cyclotron (which is only one element of a PET system), there are relatively few PET facilities available throughout the world. (At present, it is estimated that there are only about 20 PET facilities in the United States, and about 60-70 worldwide.) Only the largest hospitals are able to afford, support and staff such systems. Thus, the benefits of PET remain available to relatively few. What is needed therefore is a PET system that is more affordable and accessible to a larger number of patients and doctors.
There are numerous disadvantages of existing low energy cyclotron-based PET systems. For example, some of the radionuclides are produced using a proton beam, while others are produced using a deuteron beam, therefore some beam switching apparatus is required. While such beam switching apparatus is well known in the art, it adds to the complexity and expense of the system. Further, large amounts of power are required for such systems to operate (e.g., the proton/deuteron cyclotron typically requires 100 kW of power to operate). Also, such systems require enriched target materials if the desired radionuclides are to be efficiently produced by the proton/deuteron beam. Such enriched target materials are not readily available, and are costly to produce. Still further, due to the inherent elliptical cross sectional shape of the proton/deuteron beam, the efficient utilization of the beam in a circular target chamber is made more difficult. Moreover, due to the secondary neutrons that are naturally produced from the proton/deuteron irradiation process, thick shields must be built around the target area to confine such neutron radiation. It is not uncommon, for example, for the target chamber of such systems to be surrounded by concrete walls that are a minimum of four feet thick. This shielding, coupled with the mass and weight associated with the other elements of the system, particularly the cyclotron, results in a system that weighs on the order of 300 tons. Such heavy systems can only be installed on a ground or basement floor, thereby severely restricting those facilities where a cyclotron-based PET system could be installed.
All of the above factors combine to make the proton/deuteron cyclotron-based PET systems very expensive to build, operate and maintain. As has been indicated, such expense disadvantageously limits the number of PET systems that are built and operated, thereby making the cyclotron-based PET systems generally inaccessible and/or unavailable to many patients, hospitals and doctors. What is needed, therefore, is a radioisotope production system which can produce sufficient quantities of all of the radioisotopes of interest (.sup.18 F, .sup.11 C, .sup.15 O, .sup.13 N) and minimize some or all of the disadvantages discussed above for existing systems. The present invention advantageously addresses this need.